Molecular oxygen plays fundamental roles in the biological oxidative metabolism required for efficient conservation of energy. Therefore, the interactions of oxygen and related ligands with hemoproteins like myoglobin, hemoglobin, cytochrome oxidase cytochrome P-450, and hydroperoxidases constitute the elementary steps in transport, storage and reduction of this ubiquitous oxidant. Therefore, the specific aims of this project sharply focus on the physiological and biomedically vital aspects of oxygen delivery to and utilization in tissues. Furthermore, carbon monoxide and nitric oxide derivatives are known air pollutants. Hemoprotein nitric oxide complexes are frequently observed as intermediates and/or byproducts of nitrogen fixation, metabolism of nitrogenous carcinogens. The proposed studies of the modes of interaction of various hemoproteins with oxygen and related gaseous ligands will provide incisive information regarding the structural and electronic mechanism of interactions of hemoproteins with these ligands. We are measuring transient absorption spectra of various hemoprotein-ligand complexes after laser irradiation at time scales of pico- to microseconds to determine the amounts of geminate and deoxy states upon irradiation at ambient temperature and to estimate apparent quantum efficiencies of these systems. In addition, photolysis of the same systems will be performed at cryogenic temperatures to measure apparent quantum efficiencies to correlate them as a function of temperature and molecular structure.